1. Field of the Invention
The present disclosure relates generally to a voltage reference and, more particularly, to a programmable voltage reference.
2. Description of the Related Art
Today, systems, such as battery-powered systems, are usually designed to enter a low-power mode when the systems are not being utilized. When in the low-power mode it is desirable for the systems to consume a relatively small amount of power. In systems that utilize voltage references, it is desirable for the voltage references to be designed to consume a relatively small amount of power during normal operation, as well as when the systems are in a low-power mode. Voltage references are used in a variety of different applications. For example, analog-to-digital converters (ADCs), digital-to-analog converters (DACs), oscillators, flash memories, and voltage regulators usually require a voltage reference that is relatively insensitive to temperature, power supply, and load variations. The resolution of an ADC or a DAC, for example, is generally limited by the precision of an associated reference voltage over a power supply voltage range and operating temperature range.
Traditionally, bandgap voltage references have employed bipolar junction transistors (BJTs) to generate a relatively temperature independent reference voltage. In general, bandgap voltage references exhibit a relatively high power supply rejection ratio (PSRR) and a relatively low temperature coefficient. To reduce power consumption of integrated circuits (ICs), many IC designers have migrated from bipolar to complementary metal-oxide semiconductor (CMOS) processes. While bipolar CMOS (BiCMOS) processes may be used in the design of a bandgap voltage reference, BiCMOS devices are relatively expensive, as compared to CMOS devices. Moreover, bandgap voltage references have usually employed ratiometric related resistors. In a bandgap voltage reference, in order to provide for relatively low current, one resistor of the bandgap voltage reference is typically many times the size of another resistor. It should be appreciated that larger area resistors increase an area of an associated IC which, in turn, increases the cost of the associated IC.
U.S. Patent Application Publication No. 2006/0001412 (hereinafter “the '412 application”) discloses a voltage reference that is fabricated exclusively using CMOS processes. The voltage reference of the '412 application employs a current generator that provides a proportional-to-absolute-temperature (PTAT) current. A stack of serially coupled metal-oxide semiconductor field-effect transistors (MOSFETs) is coupled between the current generator and a common point, i.e., ground. The stack of MOSFETs have a transimpedance which has a temperature coefficient that is opposite in polarity to a temperature coefficient of an internal resistance of the current generator.
In the following detailed description of exemplary embodiments of the invention, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific exemplary embodiments in which the invention may be practiced.